Particular embodiments generally relate to wireless transmitters.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
FIG. 1 depicts a conventional transmitter 100. Differential in phase (I) and quadrature (Q) signals may be processed through separate channels. For example, the I and Q signals are input into digital programmable gain amplifiers (DPGAs) 102a/102b, which amplify the signals. Digital-to-analog converters (DACs) 104a/104b convert the digital I and Q signals to analog. The I and Q signals are then input into low pass filters (LPFs) 106a/106b, which provide attenuation of component noise, quantitization noise, and also provide gain.
Upconverters 108a/108b receive the I and Q signals from low pass filters (LPFs) 106a/106b. Also, a synthesizer 110 generates a local oscillator (LO) signal. Frequency divider/LO generator 112 then generates the I version for the LO signal (LO I) and Q version for the LO signal (LO Q). The LO I signal is sent to upconverter 108a and the LO Q signal is sent to upconverter 108b. Upconverters 108a and 108b upconvert the I and Q signals (at the baseband) to differential radio frequency (RF) signals. The differential RF signals output from upconverter 108a and upconverter 108b are summed through current summing circuitry 114 and converted to a single-ended output through a balun 116.
Balun 116 outputs the RF signal to power amplifier (PA) buffers 118 (or pre-power amplifiers). Each PA buffer 118 may be used for a wireless band, such as second generation (2G) high band (HB), 2G low band (LB), third generation (3G) HB, 3G HB/LB, and 3G LB. PA buffers 118 are used to drive external power amplifiers that are off of an integrated chip (IC). The filtering and linearity that is required by wireless applications is often not sufficiently provided by low pass filters 106.
FIG. 2A depicts a conventional differential PA buffer 118. Upconverters 108a/108b include Gm transistor pairs 202a/202b and mixers 204a/204b, respectively. The baseband I and Q signals are received at Gm transistor pairs 202a and 202b, respectively. Gm transistor pairs 202a/202b convert a voltage to a current.
Mixer 204a and mixer 204b receive the LO I and LO Q signals, respectively, and upconvert the I and Q signals to differential RF signals. The differential RF signals are combined in cascode transistor pair 206. The combined RF signals are then alternating current (AC) coupled through AC coupling capacitors 208a and 208b to PA buffer 118. For example, PA buffer 118 includes a differential pair of transistors 210 and transistors 211a and 211b. PA buffer 118 buffers the signal and outputs a differential signal to balun 116. Balun 116 then outputs the RF signal to a power amplifier at Pout.
FIG. 2B depicts a conventional single-ended PA buffer 118. Gm transistor pairs 202a/202b, mixers 204a/204b, and cascode transistor pair 206 operate similarly as described with respect to FIG. 2A. The differential RF signal from cascode transistor pair 206 is output to balun 116. The single ended output of balun 116 is then AC coupled through an AC coupling capacitor 208 to PA buffer 118. PA buffer 118 includes a first transistor 212a and a second transistor 212b that buffer the signal. A single-ended output is then output to the power amplifier.
In both examples in FIGS. 2A and 2B, PA buffers 118 add noise and distortion, which affects the linearity of the signal.